An MRAM is a promising nonvolatile memory from the perspective of high integration and high speed operation. In an MRAM, magnetoresistance elements exhibiting a “magnetoresistance effect” such as a TMR (tunnel magnetoresistance) effect are used. In a magnetoresistance element, a magnetic tunnel junction (MTJ) is formed, in which a tunnel barrier layer is sandwiched by two ferromagnetic layers, for example. One of the two ferromagnetic layers is a magnetization fixed layer (or pinned layer) having a fixed magnetization direction, and the other is a magnetization free layer (free layer) having a reversible magnetization direction.
It is known that the resistance value (R+ΔR) of an MTJ for the case where the magnetization directions of the pinned and free layers are “antiparallel” to each other is larger than the resistance value (R) for the case where they are “parallel” to each other. The MRAM uses magnetoresistance elements each having an MTJ as memory cells, and stores data in a nonvolatile manner by using the variations in the resistance values. For example, the antiparallel state is associated with data “1”, whereas the parallel state is associated with to data “0”. Data write onto a memory cell is performed by reversing the magnetization direction of the magnetization free layer.
One of the most traditional methods for writing data onto an MRAM is to reverse the magnetization of a magnetization free layer by a current magnetic field. In this writing method, however, the reversal magnetic field necessary to reverse the magnetization of the magnetization free layer increases in almost inverse proportion to the memory cell size. That is, the write current increases as the size of the memory cell is miniaturized. This is not preferable in terms of providing a highly integrated MRAM.
As a writing method that suppresses the increase in the write current caused by the miniaturization, a “spin transfer method” is proposed (See Japanese Patent Application Publication No. P2005-93488A (Patent literature 1), for example). In the spin transfer method, a spin-polarized current is injected into a ferromagnetic conductor, and the magnetization is reversed by the direct interaction between spins of conduction electrons of the current and the magnetic moment of the conductor. This phenomenon is referred to as spin transfer magnetization switching. The write operation based on the spin transfer method is appropriate to realize a highly integrated MRAM because the write current decreases as the size of the magnetization free layer is decreased.
U.S. Pat. No. 6,834,005 (Patent literature 2) discloses a magnetic shift register using spin transfer. This shift register utilizes a domain wall in magnetic material to store information. In a magnetic material divided into a number of regions (magnetic domains) by constrictions and the like, a current is injected through the domain walls, and the domain walls are moved by the current. The direction of magnetization in each of the regions is treated as record data. Such a magnetic shift register is used to record large amounts of serial data, for example.
Domain wall motion type MRAMs using such domain wall motion by spin transfer are disclosed in Japanese Patent Application Publication No. P2005-191032A and International Application No. WO2005/069368 (Patent literatures 3 and 4).
The MRAM disclosed in Japanese Patent Application Publication No. P2005-191032A is provided with: a magnetization fixed layer having a fixed magnetization; a tunnel dielectric layer laminated on the magnetization fixed layer; and a magnetization recording layer laminated on the tunnel dielectric layer. Since the magnetization recording layer includes both of a portion having a reversible magnetization direction and a portion having a magnetization direction which is not substantially changed, the magnetization recording layer is referred to as so, instead of the magnetization free layer. FIG. 1 illustrates the structure of the magnetization recording layer. In FIG. 1, the magnetization recording layer 100 has a linear shape. Specifically, the magnetization recording layer 100 includes: a junction portion 103 overlapping the tunnel dielectric layer and the magnetization fixed layer; constriction portions 104 adjacent to both ends of the junction portion 103; and a pair of magnetization fixed portions 101 and 102 respectively formed adjacent to the constriction portions 104. The pair of magnetization fixed portions 101 and 102 are provided with fixed magnetizations respectively having opposite directions to each other. Each of the magnetizations of these magnetization fixed portions is fixed by, for example, an exchange bias magnetic field formed by laminating an antiferromagnetic layer thereon. Further, the MRAM is provided with a pair of writing terminals 105 and 106 electrically connected to the pair of magnetization fixed portions 101 and 102. Through the writing terminals 105 and 106, a write current flows through the junction portion 103, the pair of constriction portions 104, and the pair of magnetization fixed portions 101 and 102 of the magnetization recording layer 100. The constriction portion 104 functions as a pinning potential for the domain wall, and information is retained depending on whether the domain wall is present in the left or right constriction portion, or depending on the magnetization direction of the junction portion 103. The direction of domain wall motion is controlled by the direction of the write current.
In the MRAM disclosed in WO2005/069368, a step is used as means to form the pinning potential. FIG. 2 illustrates the structure of the magnetization recording layer in the MRAM. In FIG. 2, the magnetization recording layer 100 includes three regions respectively having different thicknesses. Specifically, the magnetization recording layer 100 includes a thickest first magnetization fixed portion 101, a second thickest second magnetization fixed portion 102, and a thinnest junction portion 103 arranged between them. In FIG. 2, step structures provided at boundaries between the junction portion 103 and the magnetization fixed portions 101 and 102 function as the pinning potentials. This allows a domain wall 112 to move between the two step structures by applying a current. It should be noted that, in International Application No. WO2005/069368, magnetic semiconductor having anisotropy perpendicular to the film surface thereof is used as the magnetization recording layer, and the current necessary for the domain wall motion is as small as 0.35 mA. In practice, a tunnel dielectric layer and a magnetization fixed layer are arranged over the junction portion 103, but not illustrated in FIG. 2.
In a domain wall motion type MRAM, the magnetization directions of the two magnetization fixed portions of the magnetization recording layer should be directed in antiparallel to each other. In the following, the step of directing the magnetizations of the two magnetization fixed portions in the directions antiparallel to each other by, for example, applying an external magnetic field having an appropriate magnitude is referred to as “initialization”. Patent literature 3 does not refer to a method for directing the magnetizations of the two magnetization fixed portions in the directions antiparallel to each other.
International Application No. WO2005/069368 discloses that the initialization by an external magnetic field after deposition is achieved by making use of the difference in the coercive force between the first magnetization fixed portion 101 and the second magnetization fixed portion 102. Specifically, WO2005/069368 discloses that, the difference in coercive force is provided by making the thicknesses of the first magnetization fixed portion 101 and the second magnetization fixed portion 102 different from each other. Since the magnetization is unlikely to be reversed as the thickness of the magnetic layer is increased, a domain wall can be introduced at the boundary between the first magnetization fixed portion 101 and the junction portion 103 by applying a magnetic field that reverses the magnetizations of the second magnetization fixed portion 102 and junction portion 103 but does not reverse the magnetization of the first magnetization fixed portion 101.